1. Field of the Invention
Embodiments of the present invention are directed in general to novel aluminate-blue phosphors (herein referred to as blue phosphors). Specifically, embodiments of the present invention are directed to use of the novel aluminate-based blue phosphors in white light illumination systems, display applications as in back lighting in liquid crystal displays (LCD's), plasma display panels (PDP's), and cathode ray tube (CRT) displays, signal lights, and pointers.
2. State of the Art
It has been suggested that white light illumination sources based wholly or in part on the light emitting diode will likely replace the conventional, incandescent light bulb. Such devices are often referred to as “white LEDs,” although this may be somewhat of a misnomer, as an LED is generally the component of the system that provides the energy to another component, a phosphor, which emits light of more-or-less one color; the light from several of these phosphors, possibly in addition to the light from the initial pumping LED are mixed to make the white light.
Nonetheless, white LED's are known in the art, and they are relatively recent innovations. It was not until LED's emitting in the blue/ultraviolet region of the electromagnetic spectrum were developed that it became possible to fabricate a white light illumination sources based on an LED. Economically, white LED's have the potential to replace incandescent light sources (light bulbs), particularly as production costs fall and the technology develops further. In particular, the potential of a white light LED is believed to be superior to that of an incandescent bulb in lifetime, robustness, and efficiency. For example, white light illumination sources based on LED's are expected to meet industry standards for operation lifetimes of 100,000 hours, and efficiencies of 80 to 90 percent. High brightness LED's have already made a substantial impact on such areas of society as traffic light signals, replacing incandescent bulbs, and so it is not surprising that they will soon provide generalized lighting requirements in homes and businesses, as well as other everyday applications.
In general, there have been three general approaches to making white LEDs. One is to combine the output from two or more LED semiconductor junctions, such as that emitted from a blue LED and a yellow LED, or more commonly from red, green, and blue (RGB) LEDs. No phosphors are used in this first approach. The second approach is called phosphor conversion, wherein light from a blue emitting LED semiconductor junction is combined with light obtained from a phosphor excited by the blue LED. In this second situation, some of the photons are down-converted by the phosphor to produce a broad emission centered on a yellow frequency; the yellow color then mixes with other blue photons from the blue emitting LED to create the white light. In a third approach, an LED that emits light in the substantially non-visible ultraviolet (UV) portion of the electromagnetic spectrum is used to excite a blue phosphor and at least one other phosphor, which typically is or includes a yellow phosphor.
Phosphors are widely known, and may be found in such diverse applications as CRT displays, UV lamps, and flat panel displays. Phosphors function by absorbing energy of some form (which may be in the form of a beam of electrons or photons, or electrical current), and then emitting the energy as light in a longer wavelength region in a process known as luminescence. To achieve the required amount of luminescence (brightness) emitted from a white LED, a high intensity semiconductor junction is needed to sufficiently excite the phosphor such that it emits the desired color that will be mixed with other emitted colors to form a light beam that is preceived as white light by the human eye.
In many areas of technology, phosphors are zinc sulfides or yttrium oxides doped with transition metals such as Ag, Mn, Zn, or rare earth metals such as Ce, Eu, or Tb. The transition metals and/or rare earth element dopants in the crystal function as point defects, providing intermediate energy states in the material's bandgap for electrons to occupy as they transit to and from states in the valence band or conduction band. The mechanism for this type of luminescence is related to a temperature dependent fluctuation of the atoms in the crystal lattice, where oscillations of the lattice (phonons) cause displaced electron to escape from the potential traps created by the imperfections. As they relax to initial state energy states they may emit light in the process.
The blue phosphors that have been used in the past in conjunction with near-UV radiation sources have typically been divalent europium (Eu2+) activated barium magnesium aluminate (BAM) phosphors. These blue phosphors have been used in white light systems, as well as in other applications, such as plasma display panels (PDPs) as a blue emitting component.
An example of a BAM phosphor is disclosed in U.S. Pat. No. 4,110,660, where a blend containing BaF2, LiF, Al(OH)3, and Eu2O3 was fired in a hydrogen atmosphere in the temperature range of 1400 to 1650° F. for a period of 3 to 6 hours. Another blue phosphor has been described in U.S. Pat. No. 4,161,457 to K. Takahashi. This particular phosphor is represented by the formula aMgO.bBaO.cAl2O3.dEuO, wherein a, b, c, and d are numbers which satisfy the condition a+b+c+d=10, and wherein 0<a≦2.00; 0.25≦b≦2.00; 6.0≦c≦8.5; 0.05≦d≦0.30.
Other blue phosphors which have been described in the art are exemplified by the lanthanum phosphate phosphors that use trivalent Tm as an activator, Li+ and an optional amount of an alkaline earth element as a coactivator, as disclosed by R. P. Rao in U.S. Pat. No. 6,187,225. Such exemplary blue phosphors may be represented by the formula (La1−x−zTmxLiyAEz)PO4, where 0.001≦x≦0.05; 0.01≦y≦0.05; and 0≦z≦0.05. more specifically, blue phosphors employing Tm3+ and Li+ doped lanthanum phosphate phosphors, particularly when produced by the sol-gel/xerogel and solid state methods are considered to be a part of the present invention.
Blue phosphors generally represented by the formula (BaxM1−x)1−0.25yMg1−yAl10+yO17+0.25y as a host material, with Eu as an activator, and wherein M represents Ca, Sr, or Ca and Sr has been described by K. Ono et al. in U.S. Pat. No. 6,576,157, where the stoichiometric amounts of the constituent elements were represented by the relations 0.5≦x≦1, and 0.05≦y≦0.15, and where the phosphor was excited by vacuum ultraviolet radiation.
Multiphase structured Eu2+ activated La, Mg aluminate phosphors have been prepared. U.S. Pat. No. 4,249,108 reveals that the starting materials La2O3, MgO, Al(OH)3, and Eu2O3 may be fired at about 1500 to 1650° C., for about 1 to 5 hours in a reducing atmosphere. Additional blue phosphors that may be used with the present green phosphors include those disclosed in U.S. Pat. No. 5,611,959, where aluminate phosphors were taught comprising at least one element selected from the group consisting of Ba, Sr, and Ca; a Eu activator; Mg and/or Zn; and optionally Mn. This phosphor may be prepared by firing the respective oxides and/or hydroxides in a reducing atmosphere at a temperature of 1200 to 1700° C. over a period of 2 to 40 hours.
What is needed in the art is an improved blue phosphor that is capable of emitting light with a higher intensity than the currently available BAM materials. It would also be desirable to have improved blue phosphors wherein minor modifications may be made in the composition to effect changes in the emitted wavelength.